1. Field of The Invention
The present invention relates to a sealant resin composition for use in the manufacture of pouches for retort packages and a sealant film.
2. Description of the Background
In recent years, the demand for retort food has grown rapidly with the development of the food service industry, and along with this, there has been increasingly severe requirements imposed on the performance of pouches for packaging retort food. Pouches are generally formed in the form of bag by using aluminum foil or film with gas barrier properties as the substrate layer and a sealant film laminated to the substrate layer, with its circumference heat-sealed. Up to the present, polypropylene having a high melting point and therefore being heat resist has been used exclusively for the material of the sealant so that such pouches will withstand the boiling conditions at the time of retort treatment for sterilization.
However, the recent trend is toward increasing the quantities, in volume or number, of contents of pouches, resulting in an increase in the size of pouches and the weight of the contents. The trend is also toward storing and distributing retort food in a frozen state. This change in the environment surrounding retort food has given rise to the necessity for increasing the mechanical strength--particularly pouch breakage resistance--in low-temperature storage. However, the conventional polypropylene sealant layer is unable to meet this requirement for higher strength at low temperatures.
On the other hand, the use of high density polyethylene in the sealant layer improves the impact resistance at low temperatures and prevents pouches from being broken due to a drop accident of frozen food, among other things. However, since high density polyethylene does not have adequate heat resistance, problems will arise, including heat fusion, heat deformation, a decline in transparency due to whitening and breakage of the sealing section, when high density polyethylene pouches are subjected to boiling or retort treatment at 100.degree. C. to 125.degree. C. This makes the practical use of high density polyethylene pouches difficult.
It is stated in the Laid-open Patent Publication 3383/1996 that it is possible to obtain a sealant film exhibiting excellent extrudability, heat resistance and low-temperature impact resistance by compounding a small amount of high density polyethylene with ethylene-a-olefin copolymer. However, the sealant film having a high low-density component content does not show adequate whitening resistance, deformation resistance or heat fusion performance when it is boiled at high temperatures.